The present invention relates to an image forming device of the thermal transfer type.
The color printer of the thermal transfer type can be cited as the image forming device of this type. In the case of this printer, two follower pinch rollers are contacted with a platen roller (driving roller) connected to the drive shaft of a motor, and a sheet of paper (which is to be subjected to the thermal transfer) is held between the platen roller and the pinch rollers at the front and last ends thereof. When printing is to be carried out, the pinch rollers are forced against the platen roller, while the ink ribbon and the thermal head are forced against the platen roller and the paper. The platen roller is rotated under this state to carry the paper and the ink ribbon is also carried. Printing is carried out for every line, that is, ink is transferred from the ink ribbon to the paper. Color areas of Y (yellow), M (magenta), C (cyan) and BK (black) repeatedly appear on the ink ribbon for the entire length of the paper. When printing of one color is finished, the ink ribbon and the thermal head are separated from the platen roller and the paper, and the platen roller is inversely rotated to carry back the paper till the first line of printing on the paper is relocated against the thermal head. Three-color printing is similarly repeated. The most important thing is to keep the printing positions of the four colors from shifting.
In the case of the conventional devices, however, the shift of the printing positions can not be avoided for the reasons cited below. The driving platen roller and the follower pinch rollers are usually made of rubber having a large friction coefficient to carry the paper without sliding. When both of these rollers are forced against each other and the thermal head is forced against the platen roller upon printing, the platen roller collapses a little to reform its shape at the points contacting the pinch rollers and the thermal head. On the other hand, the amount of the paper carried is determined by the radius and rotating angle of the platen rollers. When the platen roller collapses even a little, therefore, the radius is reduced and the conveyance of paper can not be thus exactly controlled, thereby causing the printing positions of the second, third and fourth colors to be shifted. It may be previously calculated to what degree they are reformed, but this is unpractical because roller deformation is different for every roller. Furthermore, the fact that the roller deformation is different for three contact points brings about the following drawback. Since the paper is urged against the platen rollers by the pinch roller at the front and last ends thereof and by the thermal head at the middle portion thereof upon printing, as described above, the rotating speeds at the three contact points of the paper are made different when the urging forces become different each other. The paper is therefore bent or made loose during the printing process. The bent paper skews when it is inversely carried back since the paper is not firmly wound around the platen roller.
In addition, that area on the paper on which the printing can be done becomes substantially smaller than the actual length of the paper, because the paper is urged against the platen roller by the pinch rollers at the front and last ends thereof. More specifically, those areas of the paper which are to be located between the contact point of the platen roller and thermal head, that is, the printing point and the front end pinch roller, and between the printing point and the last end pinch roller could not be used for printing.